OP I agree 100% with PP's who are telling you to grab this support with both hands, even if you're convinced your DC doesn't have Autism !
You are not an expert. Your experience is limited to your own 2 children.
The nursery are being proactive and supportive in finding out if your child does indeed have ASD, or whether they simply have a speech delay and concentration issues.
The very fact that a 3yr old isn't interested in joining in with group activities is a possible sign. It's not about them being 'more independant', it's about them not wanting to join their friends/peers at this age.
Autism has a multitude of 'symptoms/signs', most of which won't apply to your DC. But nursery are highlighting some that DO?
Getting a diagnosis or support in education is an uphill struggle, and any child who is anywhere on the spectrum will struggle and suffer in nursery/school/secondary, but what they struggle with will be individual. It's not simply the profoundly affected, but the children on the lighter end of the spectrum will actually be worst affected because everyone expects them to act 'normally' when they can't?
Things like understanding social cues, facial expressions, rules of play, turn taking, how to join in with peers etc as well as struggling with how to understand conversation tones/inflections/eye contact cues etc.
What harm do you think will become your DC by being assessed exactly? That they'll be 'labelled'? They won't be labelled if they're NOT autistic, and if they are, you'll be able to access help for them, so it's a win/win?